This nonprovisional application claims priority under 35 U.S.C. xc2xa7 119(a) on Patent Application No. 2000-320649 filed in Japan on Oct. 20, 2000, the entirety of which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a front wheel steering system for automotive three- and four-wheeled vehicles for steering right and left front wheels.
2. Description of the Background Art
Automotive three-wheeled vehicles or four-wheeled vehicles have a front wheel steering system in which the proportion of the steered angle of the front wheel is variable with respect to the steering angle of the steering handle. One of the known front wheel steering systems of this type is, for example, presented in Japanese Unexamined Patent Application Publication No. 0115442/1994 (hereinafter referred to as a background art), and entitled xe2x80x9cRUDDER ANGLE RATIO VARYING APPARATUSxe2x80x9d.
Referring to FIGS. 1 and 2 of the above reference, the background art is constructed in such a manner that by steering the steering wheel 4 (the reference numbers cited herein correspond to those in the above reference), the steering force is transmitted to the steered wheel 30 through the steering shaft 3, the ball screw mechanism 12, the conversion nut member 7, the arm 17, the lever member 16, the cylindrical portion 19, the shaft portion 18, the first gear 21, the second gear 26, the pinion 27, the rack gear 29, the rack shaft 28, the tie rod, and the knuckle arm to rotate the steered wheel 30.
In the background art described above, as shown in FIGS. 4 and 9, the first gear 21 and the second gear 26 are elliptic gears. The steering angle ratio varying means comprises first and second gears 21, 26 having a non-linear steering angle ratio in which the steered angle of the steered wheel 30 is small in the region where the steering angle of the steering wheel 4 is small. The steered angle increases with the steering angle.
In the background art described above, the first and second gears 21, 26 constituting steering angle ratio varying means are elliptic gears having a complex configuration, which may increase the cost. In addition, in order to ensure a further stable operation of the steering angle ratio varying means, preferably, the steering angle ratio varying means is stored in a case 1. The case 1 is utilized to ensure that the input and output portions of the steering angle ratio varying means is properly sealed. Accordingly, the employment of the elliptic gear in the background art may cause an increase in the cost of the steering angle ratio varying apparatus.
Accordingly, an object of the present invention is to simplify the construction of a steering angle ratio varying means in order to decrease the cost of a front wheel steering system for automotive three- and four-wheeled vehicles.
In order to achieve the object described above, according to a first aspect of the present invention a front wheel steering system for automotive three- and four-wheeled vehicles has right and left front wheels controlled by a steering shaft rotatably mounted on the head pipe of the vehicle body frame. The steering stem is disposed below the steering shaft at a location offset from the steering shaft. The steering stem is connected to the steering shaft via a link mechanism. The right and left front wheels are connected to the steering stem via the right and left tie rods and the right and left knuckle arms. The link mechanism is constructed from an upper link, a lower link, and a universal coupling for coupling the upper link and the lower link. Accordingly, so the steering shaft is disposed between the steering stem and the universal coupling when the link mechanism is viewed in the axial direction of the steering shaft.
When the link mechanism is viewed in the axial direction of the steering shaft, when the steering angle of the steering shaft is zero, the center of the steering shaft, the universal coupling, and the center of the steering stem are aligned. When viewed from the side of the vehicle body in a direction at a right angle with this line, the proportion of the distance from the universal coupling to the center of the steering stem with respect to the distance from the center of the steering shaft to the universal coupling varies in accordance with the steering angle of the steering shaft. Consequently, the proportion of the steered angle of the front wheel with respect to the steering angle of the steering shaft (steering angle ratio) varies in accordance with the steering angle of the steering shaft. In this way, the link mechanism functions to vary the steering angle ratio. This is referred to as a steering angle ratio varying function. Since the mechanism performing the steering angle ratio varying action is a link mechanism, an extremely simple construction may be achieved.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.